Quantitative analysis of trace OH in garnet and pyroxenes

David R. Bell, Philip D. Ihinger, George R. Rossman

Division of Geological and Planetary Sciences, California Institute of Technology. Pasadena, California 91125, U.S.A.



To calibrate infrared (IR) spectroscopy for quantitative analysis of trace structural OH in specific minerals, we have determined concentrations of H in pure separates of mantle-derived pyrope garnet (56 ± 6 ppm H2O by weight), augite (268 ± 8, ppm H2O), and enstatite (217 ± 11 ppm H2O) by manometry after heating the samples and extracting H2 gas under vacuum. IR spectroscopy confirmed the presence of intrinsic OH in these samples prior to extraction and indicated between 86 and 100% removal of H during the extraction procedure. The integral specific absorption coefficients of 1.39 ± 0.14 (1 sigma), 7.09 ± 0.32, and 15.6 ± 0.94 /(ppm H2O·cm2) for pyrope, augite, and enstatite, respectively, allow precise spectroscopic determination of the OH content of upper mantle garnets and pyroxenes to concentration levels of a few parts per million. Uncertainties in accuracy depend on mineral composition and characteristics of the OH absorption spectrum and are estimated to range between ± 10 and ± 50%.